The present invention generally relates to delay distortion suppressing systems, and more particularly to a delay distortion suppressing system for use in an asynchronous transfer mode (ATM) communication system.
In the ATM communication system, each of various kinds of information such as audio signals, image signals and data is divided into cells which have a constant length, and the cells are multiplexed via a buffer and transmitted. When the number of transmitted cells increases, the cells remain in the buffer for a longer time. On the other hand, the cells remain in the buffer for a shorter time when the number of transmitted cells decreases. Accordingly, a delay distortion is introduced in the cells which are multiplexed and transmitted. At the receiving end, a receiving buffer made up of a first-in-first-out (FIFO) buffer is provided in order to suppress or absorb the delay distortion. It is desirable that the receiving end have a large delay distortion suppressing range and that the absolute delay time is small.
FIG. 1 shows an essential part of an example of a conventional ATM communication system. A transmitting end includes a coding part 41, a cell forming part 42, a signal detecting part 43 and a multiplexing part 45 which has a buffer 44. On the other hand, a receiving end includes a demultiplexing part 46, a receiving buffer 47, a cell disassembling part 48 and a decoding part 49. The transmitting end and the receiving end are coupled via a transmission line 50, an ATM exchange and the like which are not shown.
At the transmitting end, the signal detecting part 43 detects whether an input signal is an audio signal or a modem signal. The signal detecting part 43 controls the coding part 41 and the cell forming part 42 depending on the detected result, so that the input signal is subjected to an appropriate coding process in the coding part 41 and subjected to an appropriate cell forming process in the cell forming part 42. Each cell which is output from the cell forming part 42 has a length of 53 bytes which is made up of a 5-byte header and a 48-byte information field. The cells output from the cell forming part 42 are multiplexed via the buffer 44 of the multiplexing part 45 and transmitted to the transmission line 50.
Generally, with respect to the audio signal, the cell forming part 42 judges whether or not each cell is a voiced cell which includes voice or an unvoiced cell which includes no voice, and the cell forming part 42 supplies only the voiced cells to the multiplexing part 45. When transmitting the cells, the multiplexing part 45 can add a cell number, a cell transmission time and the like to the header.
At the receiving end, the demultiplexing part 46 demultiplexes the multiplexed cells received from the transmission line 50. The demultiplexed cells are supplied to the cell disassembling part 48 via the receiving buffer 47, and each cell is disassembled into the header and the information field. The information field is supplied to the decoding part 49. The cell disassembling part 48 also judges whether or not each cell is related to an audio signal or a modem signal, and controls the decoding part 49 depending on the result of the judgement. Hence, the decoding part 49 carries out a decoding process in correspondence with the kind of information, that is, the audio signal or the modem signal.
As the number of cells input to the multiplexing part 45 from several channels increases, the waiting time of the cells in the buffer 44 becomes longer. On the other hand, when the number of cells input to the multiplexing part 45 decreases, the waiting time of the cells in the buffer 44 becomes shorter. As a result, the delay time of the cells which are multiplexed in the multiplexing part 45 and transmitted to the transmission line 50 changes, and a delay distortion of the cells occurs. When such a delay distortion occurs for the audio signal cells, the quality of the reproduced audio signal at the receiving end deteriorates. Accordingly, the receiving buffer 47 is provided on the receiving end so as to suppress the delay distortion.
The receiving buffer 47 is made up of a FIFO memory having a memory capacity which is, for example, twice the delay distortion suppressing range. The cell which arrives first is written at the center of the FIFO memory and is read out by successively shifting the cell within the FIFO memory. If the second cell arrives at the time a reference delay time elapses, this second cell likewise is written at the center of the FIFO memory. If the second cell arrives after the reference delay time elapses, this next cell is written on the output side from the center of the FIFO memory. On the other hand, if the next cell arrives before the reference delay time elapses, this next cell is written on the input side from the center of the FIFO memory. As a result, the delay distortion is suppressed when the cells are output from the FIFO memory, that is, the receiving buffer 47.
However, in order to suppress the delay distortion, the FIFO memory requires a memory capacity which is twice the delay distortion suppressing range. However, when the delay distortion suppressing range is set large, the cells remain in the FIFO memory for a relatively long time and there is a problem in that the absolute delay time becomes large.
On the other hand, there is active research in broad-band integrated services digital networks (B-ISDNs) which enables communication of broad-band information such as images. The ATM is suitable for use in this broad-band ISDN. In this case, the cells are multiplexed at each node within the network, but generally, the cells are delayed if congestion occurs on the line. A difference inevitably occurs among the delay times of the cells, thereby causing a delay distortion.
FIG. 2 shows an essential part of an example of a conventional ISDN employing the ATM. In FIG. 2, those parts which are basically the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. In this example, a terminal 510 is telephone set, but various other kinds of terminals may be used as the terminal 510. The multiplexed cells are transmitted to the transmission line (not shown) via a switch 520.sub.1 of an ATM switch 520, and this ATM switch 520 carries out a switching operation so that the multiplexed cells are transmitted to an ATM switch (not shown) which is connected to a destination terminal (not shown) via the transmission line. On the other hand, the multiplexed cells which are received from the transmission line are supplied to the demultiplexer part 46 via a switch 520.sub.2 of the ATM switch 520.
After the cells are generated in the cell forming part 42, the cells are delayed when congestion occurs in the path, which path includes the multiplexing part 45, the ATM switch 520 and the transmission line, and particularly in the multiplexing part 45, which includes the buffer, and in the ATM switch 520. In this case, the cells in most cases arrive at the receiving end with different delays, and such different delays are often referred to as the delay distortion. The voice cannot be reproduced correctly if the delay distortion occurs, and there is a problem in that the contents of the communication cannot be understood by the listener when the delay distortion occurs.